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One year ago, Republicans had every reason to believe that they were poised to net at least four Senate seats in November and gain control of the chamber in the next Congress. A month ago, on the other hand, Democrats had reason to be confident that even if they lost a seat or two, their party would more likely than not retain control of the Senate in the next Congress.

But now, with the presidential race tight and polls contradictory, this year’s fight for the Senate is anything but clear.

The GOP needs three Senate seats to organize the chamber if Mitt Romney wins the White House and four Senate seats if President Barack Obama wins re-election.

Democrats are still likely to pick up the Maine open seat, and Massachusetts remains a problem for the GOP.

While Sen. Scott Brown (R) continues to run almost even with challenger Elizabeth Warren (D) in some public and private polling, voters across the country are increasingly likely to cast votes for candidates in the party they traditionally favor. That’s a problem for Brown in Democratic Massachusetts, just as it is for former WWE CEO Linda McMahon (R) in Connecticut, former state Attorney General Heidi Heitkamp (D) in North Dakota and even for Sen. Jon Tester (D) in Montana.

Democrats hope to swipe GOP-held seats in Nevada, Indiana and Arizona, and all three remain competitive.

But appointed Sen. Dean Heller (R) appears to have a slight advantage over Democratic Rep. Shelley Berkley (though her pollster, Mark Mellman, disputes that) in Nevada, and state Treasurer Richard Mourdock (R) in Indiana and Rep. Jeff Flake (R) in Arizona also are very slight favorites for November. Mourdock benefits from the same trend that should help Warren in Massachusetts.

If the two New England seats go Democratic but the other three seats remain in GOP hands, then Republicans need to take over five or six Democratic seats, not three or four, a significantly more difficult task.

Retiring Sen. Ben Nelson’s (D-Neb.) seat will flip to the GOP. North Dakota remains a tossup, but it isn’t unreasonable to believe that Romney’s strength will help tip the state’s Republican voters slightly toward Rep. Rick Berg (R), no matter how much they like Heitkamp personally.

If both seats go Republican, the GOP needs three or four seats to control the next Senate, with three states pure tosssups — Montana, Virginia and Wisconsin — and the longer shots in play — Connecticut, Ohio and Florida.